Chemistry and Problems of Industrial Water Part One
1.1.3.1 Theoretical Meaning
The ratio defi ned as the solubility product is the factor which determines precipi-tation or solubility:
Ksp
n m
n m
M An
=
[
MAn][ ]
[ ]
+ −
−
It is the equilibrium constant of the reaction MAnn m− ↔Mn++Anm−.
Analyzed in terms of precipitants, the solubility product gives the quantity of substance in solution at equilibrium . Hence, the solubility product determines the precipitating action. For this reason, K sp is also referred to as the instability constant [20] because its value becomes higher as salts become more dissociated,
that is as salts become less stable. Solubility product (or instability constant) is reported with a negative exponential. That of calcium carbonate, for instance, is
Ksp
Ca CO
=
[
CaCO][ ]
[ ] = ⋅
+ −
2 − 3 3
1 7 10. 8
Conversely, when the solubility product is related to complexes of sequestration, the aforementioned ratio is inverted so as to emphasize the thermodynamic sta-bility of the soluble complex [20] . The inverse ratio is known as the stasta-bility constant and is reported with a positive exponential. The stability constant for Ca - EDTA, for instance, is [13]
Kst
Ca EDTA Ca EDTA
=
[ ]
[ ][ ]
= ⋅−
+ −
2
2 4
3 9 10. 10
It shows the equilibrium constant of the reaction Sequestrant Metal Ion+ ↔Complex.
Stability constants are usually written in logarithmic form for ease of handling.
Examples of stability constants are reported in Tables 1.13 and 1.14 . 1.1.3.2 Practical Meaning
The competition between precipitants and sequestrants is one of the most impor-tant aspects of the detergency process. Detergency is often satisfactory or not according to the winner of this competition. The stability constant proves to be one of the parameters that predicts in advance which competitor will be the winner among precipitants and sequestrants. The complexing ability of a seques-tering agent must fi ght with the insolubilizing power of a precipitant, from which it attempts to remove the metal. As a consequence, it is possible to deduce whether a precipitant is controlled by a sequestrant simply by comparing its solubility product with the stability constant of the sequestrant. Only the anions with higher stability constants will be primarily and permanently linked to the metal. In order to graphically show the concept, Figure 1.18 , reported from Cutler and Davis [14] , explains the competition well.
Table 1.13 Examples of stability constants.
a) HEDP ATMP EDTMP DTPMP STP PBTC a
Ca 2+ 5.74 6.68 9.33 7.11 5.36 4.4
Mg 2+ 6.39 6.49 8.63 6.40 5.81 5.6
Fe 3+ 33 28.9 19.6 – – 13.3
Cu 2+ 19 13 18.95 19.47 8.70 10.1
a) Adapted from Ref. [13] .
Table 1.14 Examples of stability constants.
a) NTA EDTA DTPA HEDTA
Ca 2+ 6.4 10.6 10.8 8.2
Mg 2+ 5.5 8.8 9.3 7.0
Fe 3+ 15.9 25.0 28.0 19.8
Cu 2+ 12.9 18.7 21.4 17.5
Al 3+ 11.4 16.5 18.7 14.4
a) Adapted from [48] .
HEDP, Hydroxyethylidenediphosphonic acid; EDTA, Ethylenediaminotetracetic acid;
ATMP, Aminotrimethylenephosphonic acid; NTA, Nitrilotriacetic acid; EDTMP,
Ethylendiaminotetramethylenephosphonic acid; DTPA, Diethylenetriaminopentacetic acid; DTPMP, Diethylentriaminopentamethylenephosphonic acid; HEDTA, Hydroxyethylenediaminotriacetic acid;
STP, Sodium tripolyphosphate; PBTC, Phosphonobutanetricarboxylic acid.
Any sequestering agent shown on this fi gure dissolves all the precipitates located higher on the scale [14] . It means, for example, that EDTA is the only anion able to compete with stearate for calcium. For this reason, it is thoughtless always to use the same sequestrants in detergents and hope for good cleaning results. A deep knowledge of the chemistry of both sequestrants and precipitants is essential.
Soap - based lubrication is a typical example of the aforementioned competition.
This competition was fi rst observed during the study of the solubility of calcium Figure 1.18 Competition between complexing agents and precipitants.
Table 1.15 Example of stability constants.
Ca - EDTA 3.9 × 10 10
Calcium oxalate 3.8 × 10 8
Cu - EDTA 2.4 × 10 18
Cu - Sulfi de 1.2 × 10 44
Fe(OH) 3 1.6 × 10 36
Fe - Pentapolyphosphate 3.2 × 10 6 [12]
soaps (salts of linear carboxylic acids) [49] . The higher the stability constant, the more effective is the competition, as seen from inspection of the constants in Table 1.15 [13] . EDTA will be able to keep calcium oxalate soluble but will not prevent the precipitation of copper sulfi de. Polyphosphates do not keep iron soluble in a strong caustic medium [12] , but they will use their dispersing capacity in preventing the basic iron salt from sedimenting and scaling. The success of sodium tripolyphosphate ( STP ) as a scale prevention agent proves that the simple description of the stability constant as the reverse of the solubility product does not exactly represent what really happens in practical scale prevention. While the solubility product is only related to the quantity of dissolved salt, the stability constant is connected to a soluble complex. In terms of chemistry, salt and complex are different in structure [50] . Unlike salts, complexes change their structure according to the medium and keep their solubility. In many cases a suitable concentration of sequestrants (ionic strength) cannot be determined by a simple comparison between stabilities. Moreover, scale prevention has often to be attributed to a sort of complex between sequestrant and insolubilized salt rather than between sequestrant and metal ion. In other words, deposit preven-tion depends on the synergism between sequestrapreven-tion, dispersion, and suspen-sion. Furthermore, scale control is often achieved by preventing adhesion rather than by keeping ions soluble. Dispersion and suspension confi rm their crucial importance in detergency. They assist and complete the ability of a sequestrant to form the overall framework of the complexes [51, 52] . Going back to Table 1.15 , polyphosphates (STP in particular) will not keep iron hydroxide soluble and cannot re - solubilize it in caustic medium. However, STP performs sequestration by dispersion and anti - redeposition of solid molecules and is able to prevent deposits even in this unfavorable condition. Phosphonates and polyacrylates perform likewise. Since one single chemical manifesting these three properties to the full does not exist, judicious blends of sequestrants should be applied to every process.
As carefully examined by Chaberek and Martell [24] , sequestration depends on many factors. Concentration, temperature, pH, and ionic strength are parameters able to affect the stability constant. Therefore, constants are correctly compared only when determined and reported under the same conditions.
Sequestration depends on pH, whereas the stability constant (chemical behav-ior) is independent [13] of it or only indirectly connected to it. In actual fact, the stability constant is not associated with a sequestering anion and its sequestered metal but is an attribute of the complex. The stability constant remains the same over the whole range of pH within which that complex exists. If the stability constant appears to change, the type of complex is actually changing, usually as a function of pH. Every new complex is characterized by its own different stability, which is linked to a different constant.
The infl uence of hydrogen ion on replacing ligands is well known [53] . Gener-ally speaking, a decrease in pH weakens the stability of a complex, as hydrogen replaces one or more ligands of the complex. Sequestrants containing carboxylic groups prove to be the most sensitive. Their stability constants are remarkably strong when they are deprotonated and drastically change when pH decreases.
Examples with EDTA and NTA are given below.
EDTA [13]
Ca EDTA CaEDTA pH
Ca H EDTA Ca H EDTA
st
2 4 2 9
2 3
10 10
+ − −
+ − −
+ ↔ = =
+( ) ↔ ( )
K
Ksst=103 pH=7 NTA [18]
Cu NTA CuNTA pH
Cu H NTA Cu H NTA
st st
2 3 13
2 2
10 10 3
+ − −
+ −
+ ↔ = >
+( ) ↔ ( ) =
K K
. 1
102 7. 3<pH>10 3.
Conversely, the hydroxyl complexes of many metals offset their stability in alka-line conditions. The hydroxyl ion is such an effective ligand that it competes with sequestrants. For this reason, it is reported a conditional stability constant [48] . The conditional stability constant takes into consideration the infl uence of both hydrogen and hydroxyl ions, being defi ned as
K K
conditional MX
X M
=α α
where α X is a coeffi cient which describes progressive protonation of the ligands with decrease in pH while α M represents the tendency of the metal to be hydroxylated.
Plotting the conditional stability constant versus pH in a graph, it is possible to identify the peak of performance for each sequestrant toward the metal ions.
Examples are given in Figure 1.19 [48] .
The hydroxyl - hydrogen competition is clearly evident from the change in stabil-ity of the complex as a function of pH. The value of K conditional also indicates the peak of activity where the sequestrant shows the best control on each single metal.
When a solution contains equal concentrations of two or more metal ions, sequestrants will form complexes preferentially with the metal ion giving the higher stability of the complex [18] . The reaction follows the order of priority and virtually waits until it has gone to completion with the most stable ion prior to any reaction with the second ion on the list.
Figure 1.19 Peak of performance for different sequestrants.
1.1.4 Critical p H
The literature confi rms the primary importance of the pH effect on sequestration because of the hydrogen - hydroxyl ions competition with the sequestering agents for ligands. Actually, at high pH a ligand generally exists in the fully ionized state, which provides the most effective bonds for complexing. Lowering the pH means adding protons to the ligand stepwise until there is no ionic species capable of sequestration [14] . Investigation of the sequestering power at different pH values and different concentrations of sequestrant shows two non - linear relations:
•
the increase in pH and the quantity of adherent precipitate
•
the pH giving the highest volume of scale and the concentration of sequestrant necessary to prevent it.Deposition on every surface (fl at and vertical) shows a peak between pH 8.5 and pH 10.5, diminishing above pH 12 and changing into nonadherent agglomerates freely moving in the system. Concentrations of caustic soda above 0.5%
(pH > 12.4) change the salt structure from hard crystals into hydroxylated inert fl ocs. These hydroxide fl ocs settle on fl at surfaces where they set like stone in the absence of dispersants.
Sequestrants increasingly have diffi culty in preventing precipitation within the pH range 10 – 12.5 with a peak between 11.5 and 12. Table 1.16 shows the lack of control.
This trend is successfully applied, for instance, to determine the concentration of the most suitable sequestrant for the rinse section in bottlewashing. The bottle-washer rinse is a typical system where pH progressively decreases from ca. 12 to neutrality.
Donnel and Lin [54] clearly noticed this critical range of pH in their method for the assessment of detergent builders in water hardness control.
The critical zone of pH depends both on the chemical composition of the water (hardness salts) and the concentration of the sequestrant. In both cases, the more
1.1 Variables 37
Table 1.16 Diffi culty in preventing scale within the pH range 11.0 – 12.5.
pH 10.0 11.0 11.5 12.0 12.5
ppm 100 200 300 400 100 200 300 400 100 200 300 400 100 200 300 400 100 200 300 400
ATMP − ++ +++ +++ − ++ +++ +++ − + +++ +++ − − + ++ − − ++ +++
STP − − + ++ − − − ++ − − − − − − − − − − − ++
PA a) − − − +++ − − − ++ − − − − − − − − − − − −
a) Partially neutralized polyacrylic acid 20 000 MW.
Rating: − insuffi cient + suffi cient ++ good +++ excellent hardness control.
Conditions: 40dF, 60 ° C, 20 h of rest, sequestrants adjusted at 40% of active matter. Deposit assessment on glass slides.
the water hardness or the sequestrant concentration increases, the more the peak of the critical pH moves to higher value. However, the volume of deposit decreases when the concentration of sequestrants rises. The critical pH is also revealed by the higher concentration of sequestrant required to keep the salts soluble, or, as Ashcraft affi rms [55] , an increase in pH needs higher dosages of sequestrant to stabilize hardness.
The critical pH identifi es the transition zone where scale changes from sticky adherent crystals to amorphous material. Below the critical pH, scale grows everywhere on vertical and fl at surfaces. Above the transition zone, alkalinity (OH − ) changes the precipitate into a sludge which, in time, compacts itself like marble on fl at surfaces. Dispersants keep the sedimented precipitate in sludge form. Many techniques are available in the laboratory to make the solution suit-able for every condition in the fi eld. Hundreds of detergents have been developed to answer the purpose. The graph in Figure 1.20 illustrates what happens.
The fi rst line shows the quantity of deposit on the surface after rinsing with deionized water. The second shows the concentration of sequestrant required to prevent scale. The range between pH 13.00 and pH 13.40 (rapid increase in scale) shows what happens without dispersants: the settled deposit becomes as hard as marble after lying undisturbed for a few hours.
Figure 1.20 Effect of dispersant on deposits. Conditions: 40 dF, 70 ° C and 20 h of rest. Ca determined with EDTA.
Several metals (e.g., aluminium and iron) form hydroxy - complexes that polym-erize as the pH increases. The polymer reaches its maximum size when the charge of each complex unit is zero, that is, when the number of hydroxyl groups per metal ion is equivalent to its charge. This is schematically represented in Figure 1.21 [24] .
What is described for metals can also be applied to the ligands linking metals, whether they are precipitants or sequestrants. Thus, pH affects the behavior of both ligands and metals. When complexes co - ordinate molecules of water, they partially or totally replace them with hydroxyl ions. Moreover, the metal chelate can disproportionate to give metal hydroxide and the formation of larger com-plexes, as shown in the equation for bivalent metals (Figure 1.22 ) [24] .
The metal hydroxide is usually insoluble and settles out. The change from a 1:1 to a 2:1 metal chelate compound and the disproportionation to give a free insoluble metal hydroxide can explain the partial loss in activity of some seques-trants having an insuffi cient number of donor groups to satisfy their coordination requirements, mainly when an effective ligand, such as the hydroxyl ion, increases in the system. Thus, the competition of the hydroxyl ion explains the reduced complexing effi ciency of most sequestrants at high pH ( K conditional , as in Section 1.1.3.2 ).
The pH effect on polyphosphates clearly demonstrates the presence of a critical pH between 10 and 12. As Rudy ’ s work shows [56] , the concentration of sequester-ing agents required to thoroughly soften a fi xed volume of hard water increases only within the pH range 10 – 12. Above and below this, the concentration of polyphosphates remains unchanged, as shown in Figure 1.23 adapted from Rudy ’ s investigation [24, 56] .
Detergent solutions whose pH is in the transition zone need a mixture of sequestrants carefully chosen in type and concentration to run smoothly.